This application is the US national phase of international application PCT/GB00/03405 filed 5 Sep. 2000, which designated the US.
The present invention relate to colloidal photonic crystals, to a method of growing robust large area colloidal crystals and devices produced thereby.
For the purposes of this patent, a photonic crystal shall be defined as an object whose optical properties are spatially periodic.
It has been known for some time that colloidal suspensions can be made to crystallise under certain conditions to produce colloidal crystals which exhibit interesting optical properties.
Such photonic colloidal crystals are capable of modifying the propagation of light due to the fact that the crystal structure is periodic on the scale of the wavelength of light. Accordingly, colloidal photonic crystals find applications in a variety of optical devices including optical filters and limiters. The reflective properties of colloidal photonic crystals can also be controlled offering further opportunities for exploitation in optical devices.
Bulk samples of such crystals are however usually polycrystalline and comprise many hundreds of crystals of the order of 100 microns in size which are randomly oriented. The crystals may also possess a variety of crystalline structures including face-centred-cubic (fcc), hexagonally-close-packed (hcp), and random-close-packed (rcp). These and other imperfections within the crystal impair the optical characteristics of the crystal and make the crystal unsuitable for materials applications.
An improved method for growing colloidal photonic crystals has been proposed by P. N. Pusey and B. Ackerson, in patent GB 2 260 714. The improved method reduces the imperfections in the crystal by melting and aligning the crystal into a preferred structure. Specifically, this method relates to a suspension of monosized polymer colloidal spheres and consists of aligning the colloid into a face-centred-cubic crystal structure by applying a rectilinear shearing force usually by inducing flow in the liquid.
The photonic crystals formed by this method are essentially perfect face-centred-cubic structures. The method enables single crystals to be grown over areas larger than 1 cm2.
At the end of the growth process, after sufficient shearing force has been applied to establish a substantially single crystal structure, the structure may be sealed to retain the carrier liquid. Alternatively, some form of gelling agent may be added to the carrier liquid to improve the stability of the structure or the carrier liquid may be allowed to evaporate to a leave a self-supporting structure of colloidal particles.
Shearing produces two types of single face-centred-cubic structure, one type being produced on the forward shear, the second type being produced on the reverse shear. When the shearing is stopped the colloid relaxes into a twinned face-centred-cubic structure. In the case of the twinned arrangement both forms of face-centred-cubic structure coexist within the crystal, one on top of the other.
Whilst the twinned face-centred-cubic structure exhibits useful optical characteristics, the single face-centred-cubic arrangement provides improved optical properties over the former. For example, the single face-centred-cubic structure demonstrates improved photonic band-gap properties and can be optimised to be reflective for a large range of angles of incident radiation and polarisation angles (for incident radiation within a limited wavelength range); see for example Yablonovitch et al, xe2x80x9cThree-dimensional photonic band structurexe2x80x9d, [(Yablonovitch, E., Gmitter, T. J., Leung, K. M., Meade, R. D., Rappe, A. M., Brommer, K. D., Joannopoulos, J. D., xe2x80x9cThree-dimensional photonic band structurexe2x80x9d, Opt. and Qu. Elect., 24, S273, 1992] and references therein. The single face-centred-cubic structure therefore offers a greater potential for high quality optical devices but cannot be made by the aforementioned linear shearing method because of the tendency of the crystal to relax into the twinned face-centred cubic structure.
Further limitations of current colloidal photonic crystals relate to the physical properties of the crystals. Current colloidal photonic crystals are relatively fragile and lack permanence, largely due to the fact that the crystalline layers are merely held in place between rigid parallel plates. With reference to configurations in which the carrier fluid is retained within the colloidal crystal, the sealing can become compromised allowing unwanted evaporation of carrier liquid leading to degradation of the crystalline structure. Further, the physical dimensions of the crystals remain relatively small precluding widespread adoption of colloidal photonic crystals in optical applications.
It is an object of the present invention to provide an improved method for producing robust large area colloidal photonic crystals.
According to the present invention, a method of growing an essentially perfect colloidal photonic crystal exhibiting a single face-centred-cubic structure comprises the steps of:
preparing a suspension of monosized colloidal spheres having a volume concentration that produces spontaneous local crystallisation in a suitable dispersion medium,
inserting the colloidal suspension into a gap between two substantially parallel surfaces,
subjecting the surfaces to relative oscillating motion parallel to their surfaces and,
subjecting the surfaces to a series of small linear displacements relative to each other, the displacements being parallel to their surfaces and in two dimensions, comprising the sequence of applying a linear displacement to one of the surfaces with respect to the other surface, rotating the direction in which the linear displacement is applied to the surface by substantially 120 degrees in a single constant direction and applying a further linear displacement to the surface, the sequence being repeated until the colloidal photonic crystal has been purified into a single face-centred-cubic structure.
Preferably the dispersion medium is one that can be changed from a liquid phase to a solid phase in order to fix the colloidal crystalline structure.
The direction of rotation may be either clockwise or anticdockwise.
In an another arrangement, the method of growing an essentially perfect colloidal photonic crystal exhibiting a single face-centred-cubic structure comprises the steps of:
preparing a suspension of monosized colloidal spheres having a volume concentration that produces spontaneous local crystallisation, in a dispersion medium that can be changed from a liquid phase to a solid phase in order to fix the colloidal crystalline structure
inserting the colloidal suspension into a gap between two substantially parallel surfaces, and
subjecting the surfaces to relative oscillating motion parallel to their surfaces.
In particular, the magnitude of the small linear displacements applied to the surfaces is substantially equal to the product of the diameter of the colloidal spheres and the number of crystalline layers in the crystal.
In one preferred arrangement the surfaces are displaced with respect to each other in an equilateral triangle.
The minimum volume fraction of monosized colloidal spheres is advantageously 0.49 and preferably the radius of the monosized colloidal spheres is in the range 0.01 xcexcm to 100 xcexcm.
Preferably the radius of the monosized colloidal spheres is in the range 0.05 xcexcm to 10 xcexcm.
The colloidal spheres may comprise at least one of a polymer, a nonlinear material, a magnetic material, a metal, a semiconductor, glass doped with an active dye, polymer doped with an active dye, and silica. In particular the colloidal spheres may be polymethylmethacrylate.
The material used for the dispersion medium is preferably at least one of an adhesive, a polymer, a resin, a non-linear optical material, an active optical material, octanol.
In a preferred embodiment the active optical material used for the dispersion medium is a liquid crystal material.
In one arrangement the dispersion medium is subsequently removed from the colloidal photonic crystal to leave a structure comprising colloidal spheres surrounded by an interconnecting matrix of voids. A substitute material may be subsequently introduced into the interconnecting matrix of voids surrounding the colloidal spheres. The substitute material may be at least one of a metal, a semiconductor, a nonlinear optical material, an active optical material.
In a preferred embodiment the colloidal spheres may be subsequently removed from the substitute material to produce an inverse single face-centred-cubic structure.
In a further preferred embodiment the active optical material used for the substitute material is a liquid crystal material.
Where the dispersion medium or the substitute material is a liquid crystal, means for applying an electric field to the liquid crystal material may be added to the colloidal photonic crystal.
In a preferred embodiment, the dispersion medium is an epoxy resin and the method of growing an essentially perfect colloidal photonic crystal exhibiting a single face-centred-cubic structure further comprises the subsequent step of curing the resin to form a solid interconnecting matrix between the colloidal spheres.
The curing process preferably includes at least one of exposure to electromagnetic radiation, exposure to ultraviolet radiation, chemical reaction, elevation of temperature.
At least one of the substantially parallel surfaces may comprise a substantially flexible membrane.
In a preferred embodiment bulk colloidal photonic crystal film may be produced by applying the linear displacements to the parallel surfaces by rolling means.
In a further preferred embodiment the method further includes the intermediate step of applying a detachable membrane to the internal face of at least one of the parallel surfaces prior to Introducing the colloidal suspension.
The internal surface of at least one of the parallel surfaces may be textured to promote the growth of multiple crystal domains.
The refractive index of the dispersion medium may be substantially different from the refractive index of the colloidal spheres. Further, the refractive index ratio between the colloidal spheres and the dispersion medium may greater than two.
In one preferred arrangement, the method comprises the subsequent step of removing the colloidal spheres from the solidified dispersion medium to produce an inverse single face-centred-cubic structure.
In a further preferred arrangement the method further comprises the subsequent step of introducing a substitute material into the voids created in the solidified dispersion medium by the removal of the colloidal spheres, thereby producing a substituted single face-centred-cubic structure. The substitute material may be a non-linear optical material, an active optical material or a laser dye.
In a preferred arrangement the two surfaces used to retain the colloidal suspension may be concentrically cylindrical.
In a second aspect of the invention there is provided a colloidal crystal grown according to the above method.
In one preferred arrangement the colloidal crystal forms an optical notch filter wherein the colloidal sphere radius and refractive index of the dispersion medium are selected to co-operate to reflect at least one specific wavelength and to transmit other wavelengths.
In a further preferred arrangement the colloidal crystal is incorporated in an optical device which further comprises a liquid crystal material and means for applying an electric field to the liquid crystal material, wherein a variable voltage may be applied to the liquid crystal material to change the refractive index contrast between the liquid crystal material and the colloidal spheres.